The present invention relates to a filter comprising plural thin film bulk acoustic resonators (FBARs) and more particularly, is directed to a piezoelectric filter which is manufactured using thin film technology in order to obtain a wide band-pass at high frequency.
Thin film bulk acoustic resonators (FBARs) are attractive devices since they show resonant peaks at high frequency, particularly in the MHz and GHz regions. Moreover, FBARs can be achieved in a small device size (xcx9cmm). Thus, they are considered to be useful not only as resonators, but to be applied to filters, transducers, etc., which would be installed in small, light, thin electric appliance products, such as mobile telephones.
It is well known that the resonant frequency of a piezoelectric resonator such as an FBAR can be changed with the thickness of the piezoelectric layer. Also, it has been reported that the resonant frequency of an FBAR can be changed by changing the thicknesses of other layers such as the electrodes, any underlying membrane layer or the underlying substrate. This effect is called a xe2x80x9cmass loading effectxe2x80x9d. The mass loading effect is observed particularly in MHz and GHz region. The mass loading effect is based on the fact that the resonant frequency is changed by a change in the weight in a unit area of the device. Therefore, for instance, the resonant frequency can also be changed by altering the material used for electrodes because the weight of the electrodes is changed when densities for the electrode materials are different, even if the thickness is the same.
Using the mass loading effect, it has been proposed previously that the frequency of an FBAR can be tuned by increasing or decreasing a thickness of an electrode or a membrane layer. This method has the advantage that the frequency can be tuned after making the FBAR device because it is possible in principle to increase or decrease the thickness of a top electrode. In other words, the resonant frequency can be adjusted to what is desired, by first checking the resonance of the FBAR device and then altering the electrode thickness appropriately. However, because an FBAR consists of a thin piezoelectric layer between 2 metal electrodes, it is not easy to tune the resonant frequency by altering the piezoelectric layer thickness after top electrode deposition. A method for metal ablation (such as pulsed laser illumination or ion beam etching) would need to be used.
It is known that an electric filter can be made with plural FBARs. This kind of filter can work in the MHz or in the GHz frequency regions. Generally speaking, for a band-pass filter, the rejection level for out-of-band signals relative to the pass-band improves when more FBARs are used for the filter.
FIG. 7 shows one example of a filter comprising four FBARs. The four FBARs are separated into 2 groups according to their functions in the filter. FBAR1 and FBAR2 in FIG. 7 are connected in series. Therefore they form one group. Also, FBAR3 and FBAR4 are connected in parallel and form the other group. Usually all the FBARs are prepared simultaneously and on one substrate under the same procedures. Therefore each FBAR consists of very closely matched (ideally identical) structures. In other words, the same thicknesses and the same materials are used for all the layers in each FBAR.
It is important to prepare a band-pass filter at high frequency in MHz or GHz region because those frequency regions are often used for wireless communications these days. The bandwidth of the filter is demanded to be wider or narrower sometimes. In order to make the bandwidth wider for a filter which is employing FBARs, a piezoelectric material which shows high electromechanical coupling coefficient should be applied to the FBARs because the bandwidth of each FBAR depends mainly on the electromechanical coupling coefficient. However, it is sometimes difficult to apply a material which shows high electromechanical coupling coefficient (xcexa) for FBAR because of process problems. Also, there is a limit to the electromechanical coupling coefficient. Practically, no thin film piezoelectric materials are known with xcexa greater than 0.45.
The present invention describes a wide bandpass filter which comprises plural FBARs. In detail, by this invention a wide bandpass region can be obtained by changing thicknesses of the non-piezoelectric layers in the series relative to the parallel FBARs.
The filter comprises a plurality of FBARs of which at least one FBAR is in series and one FBAR in parallel. Each FBAR comprises a plurality of layers consisting of (from lower to upper): a substrate, a dielectric layer, one or more metal layers acting as a lower electrode, a piezoelectric layer, one or more metal layers acting as an upper electrode and any top layer (optional) which might be added to effect further adjustable mass loading. This final layer can be either a conductor or an insulator.
An FBAR exhibits a series resonance and a parallel resonance at respective frequencies that are functions of the thicknesses of the layers. In the filter, it is desirable to control the thicknesses of one or several layer(s) between FBARs in series and FBARs in parallel. In this way series and parallel resonant peaks are engineered to be at different frequencies for the FBARs in series relative to those in parallel.
As a result of making a filter using those FBARs in series and in parallel, a wide band-pass filter can be obtained. In this case, the thickness of the piezoelectric layer is not supposed to be changed for the FBARs in series relative to those in parallel. The thicknesses of one or more of the following layer(s) are changed: electrodes, membrane layer, any remaining substrate, the top (optional) layer.
The theory of the device is described using FIG. 7. In FIG. 7, if all FBARs are prepared under the same condition and with layers of materials of the same type and the same thicknesses, both FBARs in series (FBARs 1 and 2) and FBARs in parallel (FBARs 3 and 4) are identical and exhibit exactly the same resonant peaks as shown in FIG. 8(a). As a result of making a filter using FBARs in series and parallel which are identical, the width of the band-pass is limited, with the frequency of the series resonance (fs) and a frequency of the parallel resonance (fp), as shown in FIG. 8(b).
On the other hand, if in FIG. 7, it is assumed that the thicknesses of the top electrodes for the FBARs in series are different from those on the FBARs in parallel while the other dimensions on all FBARs are the same, then the FBARs in series and FBARs in parallel show their resonant peaks at different frequencies, as shown in FIG. 9(a). In FIG. 9(a), the frequency of the series resonance of the FBARs in series (fs1) is set at the same frequency as the parallel resonance of the FBARs in parallel (fp2). As a result of preparing a filter using those different FBARs in series and in parallel, the bandwidth of the filter increases. This is because the width of the bandpass is limited by the frequency of the series resonance of the FBARs in parallel (fs2) and the frequency of the parallel resonance of the FBARs in series (fp1), as shown in FIG. 9(b).
There are two merits of the filter in FIG. 9(b) relative to that in FIG. 8(b). One is that the close-in rejections at fs2 and fp1 in FIG. 9(b) are deeper than those at fs and fp in FIG. 8(b). The other is that the insertion loss in the band-pass region between fs2 and fp1 in FIG. 9(b) is less than that between fs and fp in FIG. 8(b).
An important point of the proposed filter is that all FBARs both in series and in parallel comprise the same thickness of piezoelectric layer. On the proposed filter, it is essential to alter a thickness of one or several layer(s) except the piezoelectric layer between FBARs in series and FBARs in parallel. The layer(s) which show(s) the different thicknesses is/are supposed to be one or several layer(s) of electrodes, a membrane layer, any remaining substrate and so on but not the piezoelectric layer.
A method for altering the thickness of one of these layers will now be described. In order to alter the thickness of the top electrode, for instance, photolithography can be used to cover some FBARs with photoresisit, after applying the top electrode with a certain coating thickness. The thickness of the top electrode in the uncoated areas can be reduced by chemical or dry etching (e.g. ion milling and/or reactive ion etching) or can be increased by applying a further coating of metal or some other material, which can be insulating. Alternatively, by using photolithography, since a top electrode can be prepared in particular areas, top electrode areas with different thicknesses can be obtained by repeating the photolithography and coating for different device areas. In this case there is no need for electrode etching or for coating over existing top electrode areas.
Another good example of a method for controlling the thickness of the top electrodes in particular areas is described as follows. Excimer laser pulses, which can be focussed into spots of very small areas, can be used to etch the top electrodes locally. During the etching or before and after the etching, the resonant frequencies of the devices being etched can be monitored. In this case, it is very quick and precise to obtain the required frequencies for particular FBARs.
It should be noted that when attempts are made to locally reduce the thickness of the piezoelectric layer by etching, it can occur that the surface of the etched piezoelectric layer becomes rough. That rough surface may cause a loss of signal because resonance occurs in the said piezoelectric layer. Alternatively, when attempts are made to locally increase the thickness of the piezoelectric layer by coating over an existing piezoelectric layer, it is possible that the film properties of the added piezoelectric layer are changed relative to the existing one. Moreover, if attempts are made to change the resonant frequency by changing the thickness of the piezoelectric layer, it is difficult to check the resonant frequency of the device because the piezoelectric layer needs to have a top and a bottom electrode on both sides to allow the resonance to be excited electrically. This makes it hard to increase or decrease the thickness of the piezoelectric layer within such a sandwich structure between the electrodes. For these reasons, it is important to change the thickness of one or some layers other than the piezoelectric layer on FBARs in order to make the proposed filter.